There are 4 Gospels included in the New Testament. They are all formally anonymous (the author does not identify himself) but all are associated consistently with 4 authors. These attributions are consistent and widespread, and there seems to be no confusion as to who wrote the texts in the accounts of early Christians, and even later Christian authors.
This consistency is highly unusual, implying a very strong tradition of authorship for all 4 Gospels. This defies explanation if early Christian communities were not intimate connected and the authorship of each Gospels was established early.
The twist on this is that each Gospel contains a character that is ambiguously associated with the author:
Matthew (9:9), Mark (14:51-52), Luke (24:13), John (21:7)
What explains this phenomenon?